The broad aim of this research is to define the role of endosome acidification in virus penetration of host cells. There are five specific aims: 1) We believe that successful virus infection (viral replication and propagation of new virus particles) normally depends on passage of internalized virus through an acidic compartment, the endosome. We have developed subcellular fractionation techniques which allowed us to show that the isolated endosome acidifies by an ATP-dependent mechanism. Our first aim is to define the ATP-dependent mechanism of endosome acidification. 2) We have obtained, from several sources, Mammalian cell mutants that are "cross resistant" to both diphtheria toxin and animal viruses. Preliminary results implicate a defect in endosome acidification as the basis for "cross resistance". Our second aim is to verify and characterize the defect in acidification of endosomes in cross resistant mutants. 3) Certain amines have been reported to inhibit the induction of the anti-viral state by interferon. Our third aim is to test the hypothesis that this effect of amines is explained by a requirement for interferon to enter the host cell through an acidic compartment to induce the anti-viral state. 4) The Mg++ and ATP-dependent acidification of endosomes presumably depends on a Mg-ATPase. Our fourth aim is to identify the ATPase responsible for acidification of endosomes, and to purify and characterize it. 5) Our fifth aim is to develop a model system for studying inhibitors of endosome acidification for their action as inhibitors of virus infection, and inhibitors of virus infection for their effects on endosome acidification. These studies should elucidate the role of the endosome in virus penetration of host cells and may provide the framework for developing new anti-viral compounds.